This project consists of studies on the transport of vitamin B12 across bacterial cell envelopes. At present, the primary focus of this work is on the purification and properties of the B12 receptor sites which are firmly bound to the outer membrane of the E. coli cell envelope. These receptor sites, which apparently facilitate the movement of vitamin B12 across the outer membrane, also serve as receptors for bacteriophage BF 23 and colicins E 1 and E 3. An understanding of the structure of these receptor sites in relation to these various functions is a major goal. A second major goal is an understanding of the interactions of the outer membrane B12 receptor sites with the energy-coupled B12 transport system associated with the inner, or cytoplasmic, membrane.